I think the best example of a genre of music that has degraded over time is hip-hop music. In the 80's, hip-hop had a powerful message. In the 90's, hip-hop was more like poetry. Tupac and Biggie were at the forefront, and they defined the era. In the early 2000's Eminem hit the scene and blew everybody away with his talent and excellent lyrics. Now, modern hip-hop is more focused on degrading women and artists are more concerned about having a hit single than they are about telling a story.
Another genre that is suffering is alternative rock. In the 90’s alternative rock bands such as Nirvana, Hole, and Alanis Morrisette were larger than life. So many great groups came out of that period. Again, there was a definitive message behind the music. Now, it seems they are only concerned with getting radio airplay.